Historically, electrical relays and in particular power-rated relays, have been employed in switching situations where it is desired either to institute or interrupt electric current flow through a circuit. Conventionally, an electromagnetic solenoid operated relay has been used for this purpose wherein a small actuating signal current has been employed to either close or open the contacts of the power rated relay which then controls current flow from a larger current source through the relay contacts to a circuit being supplied via the relay. In the case of a latching relay, the contacts of the latching relay when set in either a closed or open circuit condition, remain in that condition until reset to the opposite condition by subsequent actuation of the electromagnetic solenoid employed to drive the larger current rated contacts of the latching relay to their opposite closed or opened condition.
Relays which use piezoelectric drive elements offer several advantages over their electromagnetic solenoid driven counterparts. Typically, a piezoelectric driven relay requires lower current and dissipates very little power in comparison to the electromagnetic solenoid driven relay. In addition, piezoelectric driven devices have a very low mass structure thereby employing less space with less weight and also possess very short actuation times. Thus, fast acting relay switching is possible with smaller and lower weight devices which also dissipate less power and hence operate at lower temperatures.
Previous attempts to provide piezoelectrically driven relays have resulted in relays having poor performance characteristics. In the case of bender-type piezoelectric direct current driven relays, the prior art devices implemented in this manner possess severe performance limitations which are founded in the trade-offs between contact force, contact separation, depolarization and the uncertainty of contact position due to creep and temperature effects which build up over a period of continued relay usage.
Prior art piezoelectrically driven relay devices have been described, for example, in U.S. Pat. No. 2,166,763 issued July 18, 1939 for a "Piezoelectric Apparatus and Circuits". In this apparatus a relay device having a piezoelectric bender-type drive member comprised by two juxtaposed piezoelectric plate elements is disclosed and is so designed that upon application of an electrical potential force between the input terminals to the device, one of the plate elements lengthens and the other shortens. As a result the bender-type drive member bends in the manner of a bimetallic thermostat and closes the contacts of a switch comprised by a fixed contact and a movable contact mounted on the piezoelectric plate elements. In this arrangement, one of the piezoelectric plate elements will have the actuating potential applied in phase with a pre-poling electric field and the other piezoelectric plate element will have the actuating signal of opposite polarity with a prepolarization field. As a consequence, with this type of device, long term depolarization of either one or both piezoelectric plate elements occurs due primarily to the depolarizing effects of the applied out of phase actuating signals. Similar objectionable characteristics are present in many of the prior art piezoelectrically driven bender-type switches and/or relay devices exemplified by the following patents: U.S. Pat. No. 2,182,340--issued Dec. 5, 1939 for "Signalling System"; U.S. Pat. No. 2,203,332--issued June 4, 1950 for "Piezoelectric Device"; U.S. Pat. No. 2,227,268--issued Dec. 31, 1940 for "Piezoelectric Apparatus"; U.S. Pat. No. 2,365,738--issued Dec. 26, 1944 for "Relay"; U.S. Pat. No. 2,714,642--issued Aug. 2, 1955 for "High Speed Relay of Electromechanical Transducer Material"; U.S. Pat. No. 4,093,883--issued June 6, 1978 for "Piezoelectric Multimorph Switches"; U.S. Pat. No. 4,395,651--issued July 26, 1983 for "Low Energy Relay Using Piezoelectric Bender Elements"; and U.S. Pat. No. 4,403,166--issued Sept. 6, 1983 for "Piezeoelectric Relay with Oppositely Bending Bimorphs".
In order to overcome the deficiencies of the known prior art piezoelectrically driven relays and switches noted above, the present invention was devised wherein an alternating current excitation signal is applied selectively to the piezoelectric drive members directly for a short period of time. Because of the alternating nature of the applied excitation signal, substantially no depolarization of the piezoelectric plate elements or long term deformation (known as creep) occurs during successive operations of the alternating current excited relaya over extended periods of usage.